1. Field of the Invention
The present invention relates to a method for forming a ball on a bonding wire and more particularly to a ball forming method in which a high voltage is applied across the tip end of a bonding wire and a discharge electrode so as to cause a discharge to take place, thus forming a ball on the tip end of the wire.
2. Prior Art
Conventionally, a fixed discharge electrode system is used for forming a ball at the end of a bonding wire. In this fixed discharge electrode system, a discharge electrode is positionally fixed. The discharge electrode is positioned as close as possible to the tip end of the wire that passes through a capillary. In addition, the discharge electrode is positioned to one side of the tip end of the wire so that the discharge electrode will not contact the capillary when the capillary is lowered.
Accordingly, in the fixed discharge electrode system, the ball is not formed when the discharge electrode is directly positioned beneath the tip end of the wire as in the case of a movable discharge electrode system. Thus, since the electric discharge is performed from the side of the wire, the ball may be distorted or an eccentric ball may be formed which deviates from the center of the wire in the direction of the wire diameter.
Japanese Patent Application Laid-Open (Kokai) No. H10-125714 (hereafter referred to as "Conventional Example 1") and Japanese Patent Application Laid-Open (Kokai) No. H10-125715 (hereafter referred to as "Conventional Example 2") disclose methods for avoiding an eccentric ball or a distorted ball in such fixed discharge electrode systems.
In Conventional Example 1, a gas is blown onto the discharge spark from the direction of the discharge electrode during ball formation, thus bending the discharge spark and causing the portion of the discharge spark that faces the wire to be positioned beneath the wire. In Conventional Example 2, a magnetic field is applied to the discharge spark, so that the portion of the discharge spark that faces the wire is bent beneath the wire.
However, Conventional Example 1 requires an auxiliary device which blows a gas onto the discharge spark, and in Conventional Example 2, an auxiliary device which applies a magnetic field to the discharge spark is required. Accordingly, both methods lead to an increase in equipment costs. Furthermore, in these prior arts, a precision control of the gas or electric field is required.